The present invention broadly relates to a new aluminum alloy having a specified range of manganese which is especially significant in making the alloy conductive to being continuously cast into sheet metal stock and the method of preparation of such sheet metal stock.
Aluminum alloy sheet metal stock is widely used in the manufacture of closures, such as bottle lids; containers, such as beverage cans; air conditioning ducts, such as a spiral duct; and heat transfer fins, such as fin stock. In order to produce such products, the thickness of the thin gauge aluminum sheet must be about 0.10 millimeters to 0.27 millimeters to conserve metallic aluminum and thereby reduce material costs. At such thin gauges, it is essential that the sheet metal stock have relatively high tensile and yield strength so that the products produced will be sufficiently strong. The sheet metal stock must also be ductile and readily formable.
Before the present invention, the aluminum alloy most commonly used to produce such sheet metal stock met the specifications of the American Society For Metals (A.S.M.) aluminum alloy specification 3003. This alloy contains 1.0 to 1.5 percent manganese. Other constituents include 0.6 percent silicon, 0.7 percent iron, 0.05 to 0.20 percent copper, and 0.3 percent zinc with the remainder aluminum. The 3003 specification aluminum alloy can be cast by conventional hot mill processes and formed into desired products, such as beverage containers. However, it has heretofore been impossible to use an alloy conforming to a specification such as 3003, in a continuous casting process to produce the desired aluminum sheet metal stock.
The first teaching of the continuous casting of aluminum alloys was described in U.S. Pat. No. 2,790,216, issued to J. L. Hunter. Today many variations on the Hunter continuous process exist. The continuous casting of metal is preferred over the more conventional methods, referred to in the aluminum industry as D.C. or direct chill, because the water cooled rolls of the continuous casting method increase the metal solidification rate. This more rapid solidification rate, obtainable with continuous methods, produces several desirable metallurgical characteristics. The primary advantage is the development of a desirable constituent particle population. The distribution of the particles is much greater and their size is much smaller than is possible with material produced at solidification rates experienced with direct chill methods. This difference in distribution can be readily seen in the microstructure when comparing direct chill with continuous processes. Importantly, the same degree of fineness and greater distribution cannot be obtained with sheet produced by direct chill as with the continuous casting process. When casting aluminum alloys containing manganese, aluminum-manganese compounds, such as MNAL6 and ALMNSI, predominate as constituents. When alloys containing percentages of manganese of 1.0 to 1.8 percent are attempted to be continuously cast into sheet metal stock for products, such as containers, the resulting cast stock possesses an excessive amount, size, an distribution of manganese-base constituent. In other words, the resulting cast stock is too hard to cold work into an acceptable sheet metal stock.
The present invention provides a manganese-aluminum alloy which is especially conducive to being continuously cast and subsequently cold worked into sheet metal stock for products. The new alloy therefore provides an optimum distribution of inter-metallic manganese based constituents required by continuous casting that was not present in those alloys utilized in the direct chill methods. As a result, the drawing and forming characteristics of the continuously cast stock from this alloy are dramatically and strikingly improved over any alloys heretofore used in the continuous cast method. In fact, products which could not previously be produced from sheet stock of continuous origin can now be readily fabricated. This represents a significant contribution to the technology of producing generalized products using continuous methods. It is expected that conversion of many existing products to this alloy will lend to noticeable improvements in fabrication results, improved quality levels, and reduced production costs. The alloy should lend itself to many heretofore unexpected or unrecognized applications.